A future cellular network has characteristics of high data transmission rate and large covering area. According to requirements specified in ITU-R M1645, the data rate must reach 1 Gbps for a user moving in low speed or being in stationary status, and 100 Mbps for a user moving in high speed.
Generally when a power of a transmitter remains constant, energy of each bit is in inverse ratio with the data rate, i.e. with an increase of data rate, the SNR Eb/No will decrease linearly, which can result in a failure reception of the receiver and therefore a covering area is reduced.
Furthermore, a working band of Third Generation Wireless Communication (3G) is 2 GHz. However, the working band available for Next Generation Network (NGN) is higher than that of 3G, for example 5 GHz. The high working band results in more path loss and is more sensitive to fading, which deteriorate the performance of the covering area.
A relay station (RS) is one of effective methods to solve the above problem. A principle of the RS is to sacrifice capacity for covering area. Because of the limit of resource orthogonality, the RS cannot transmit and receive different signals on the same frequency and code channel at the same time. That is, the transmitting and receiving of the RS must be orthogonal, using different time and frequency. Furthermore, because of the near-far effect, the RS cannot transmit and receive different signals on different code channels at the same time.
The following is a basic principle of the work of a RS with reference to the downlink of the time division duplex.
In a traditional cellular network, a base station (BS) and a user equipment (UE) can always use a slot resource allocated to the UE to transmit/receive signals. However, in a cellular network based on RS, the slot resource allocated to the UE can be divided into two parts, one for transmission from a BS to a RS and the other for transmission from the RS to a UE. Thus when a transmitting power is constant, a throughput of the cellular network based on RS is half of that of the traditional network. The above is an explanation of sacrificing capacity for covering area by using RS. The network based on RS can expand the covering area and when transmission distances are the same, it equals to reducing power loss.
In addition, the RS has advantages of low cost and easy rollout. Besides a base station equipment, a power source and a machine room, the establishment of a base station needs a fiber network that costs a lot. In contrast, the equipment of the RS is simpler than that of the base station and the RS connects with the base station through a wireless link without the need of a fiber network, which reduces the cost of network expansion.
Channel multiplexing is one important way to improve the use efficiency of each channel and the system capacity. As viewed from the system level, a channel can be multiplexed between different multi-hop users or between a multi-hop user and a one-hop user; as viewed from the link level, a channel can be multiplexed between the uplink and the downlink of one multi-hop user.
FIG. 1 is a diagram showing a principle of a first communication method in a cellular network based on RS in prior art. As shown in FIG. 1, an uplink (UL) and a downlink (DL) need to occupy independent resources. For a two-hop user, taking time division duplex as an example, all together 4 independent slots are needed and for uplink (UL1, UL2) and downlink (DL1, DL2) respectively. In this case, as shown in FIG. 2, the use efficiency of each channel is only 1 link/slot, which is very low.
In order to improve the use efficiency of each channel, an improved duplex method is proposed, as shown in FIG. 3. FIG. 3 is a diagram showing a principle of a second communication method in a cellular network based on RS in prior art. In the first slot, a signal Xd is transmitted on downlink DL1 from a base station to a RS. In the second slot, a signal Xu is transmitted on uplink UL1 from a UE to the RS, and after the RS receives and decodes the downlink signal Xd and the uplink signal Xu, it combines the uplink and downlink signals to one signal Xc by a XOR operation. In the third slot, the RS transmits the combined signal Xc to the base station and the UE through the uplink UL2 and the downlink DL2 respectively and the base station and the UE can receive the signal Xc at the same time. After receiving and decodes the signal Xc, the base station performs the XOR operation on the signal Xc and the signal Xd transmitted in the first slot by the base station and gets the signal Xu, which is transmitted to the base station from the UE. The UE performs the same operation, i.e. it performs the XOR operation on the signal Xu transmitted in the second slot by the UE and the signal Xc received in the third slot by the UE and gets the signal Xd from the base station.
As shown in FIG. 4, after the above processing, four links (DL1, UL1, DL2 and UL2) can be transmitted in three slots and the use efficiency of each channel is 4 links/3 slots. Compared with the prior art in FIG. 1, the one in FIG. 4 can improve the use efficiency of each channel effectively and thus improve the network performance.
However, applications in a space domain are not mentioned in the second communication method and when applied in the space domain, the second method is not the optimal solution in improving the use efficiency of each channel.